Method for enhancing the crystallization rate of high purity amorphous Si3 N2 powder by intimate contact with a titanium containing material

ABSTRACT

High purity, fine Si 3  N 4  powder produced by the vapor phase reaction of SiCl 4  with NH 3  is amorphous. The crystallization rate of the amorphous powder is enhanced by heating the powder while in intimate contact with a titanium containing material, for example, TiN codeposited with the Si 3  N 4  by the simultaneous reaction of TiCl 4  with NH 3 .

This application is a division of Ser. No. 526,257, filed Nov. 22, 1974,U.S. Pat. No. 4,145,244.

BACKGROUND OF THE INVENTION

This invention relates to a method for enhancing the crystallizationrate of high purity amorphous Si₃ N₄ powder, powders produced by suchmethod.

Si₃ N₄ has generated considerable interest recently as a possiblesubstitute for super alloys in applications requiring high strength atelevated temperatures and good resistance to thermal shock andcorrosion, an outstanding example of which is turbine engine components.Optimization of the physical properties of this material, particularlyhigh temperature strength and thermal shock resistance holds out thepromise of significant increases in the operating temperatures andefficiencies of turbine engines over those possible with super alloycomponents. Such optimization is dependent upon the ability to producebodies of high purity and high density.

However, highest densities are at present obtained by hot pressingpowders containing significant amounts (several percent) of grain growthinhibitors. See, for example, Powder Metallurgy, 1961, No. 8, p. 145.Thus, achieving both high purity and high density would appear to dependupon the development of pure powders having significantly enhancedreactivity over those presently available.

Copending U.S. Patent Application Ser. No. 436,432, filed Jan. 25, 1974,now abandoned, and assigned to the present assignee describes atechnique, for producing high purity fine grain Si₃ N₄ powder, basedupon the vapor phase reaction of a silicon halide compound with ammonia.This powder possesses a purity of at least 99.9 percent, an averagegrain size below 1.0 microns and an adsorbed oxygen content typicallyless than 4 percent by weight which may be subsequently reduced to lessthan 1 percent by heating the powder in dry nitrogen or othernon-reactive atmosphere. In addition, the powder is characterized asbeing in the amorphous state, and is often at least partly crystallizedby heating in a non-reactive atmosphere at a temperature within therange of about 1500° C. to 1600° C. for several hours in order toenhance the formation of the alpha polymorph of silicon nitride. Thecrystallized material is then consolidated and formed into densepolycrystalline bodies by conventional means such as mixing withappropriate binders, presintering and either cold pressing to compacts,followed by sintering the compacts to achieve densification, or hotpressing in the conventional manner.

It would be particularly advantageous in the commercial production ofsuch crystallized powders if the crystallization rate could beincreased, thereby enabling shorter heat treatment times or lower heattreatment temperatures or both, and use of less refractory or lesschemically inert containers.

SUMMARY OF THE INVENTION

In accordance with the invention, it has been discovered that thecrystallization rate of high purity fine grain amorphous Si₃ N₄ powdermay be enhanced by heating such powder while in intimate contact with atitanium containing material. As little as 0.01 weight percent titaniumin a powder mixture enables crystallization of the amorphous Si₃ N₄powder at temperatures as low as 1400° C. Such intimate contact may beachieved in one embodiment by coprecipitating titanium nitride with theSi₃ N₄ from the vapor phase. The crystallization is then carried outduring a subsequent heat treating step. In accordance with an alternateembodiment, such intimate contact may be achieved by carrying out thecrystallization heat treatment in a container of an inert refractorymaterial together with sufficient titanium to achieve diffusiontherefrom into the Si₃ N₄ powder in the amount of at least 0.01 weightpercent.

The Si₃ N₄ powders are useful in the formation of shaped bodies byeither cold pressing, casting, extrusion or other forming techniquesfollowed by sintering to densify the cold formed compact, or by hotforming, e.g., hot pressing to directly form a densified body.

DETAILED DESCRIPTION OF THE INVENTION

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims in connection withthe above description of some of the aspects of the invention.

The method of forming the silicon nitride powder by vapor phase reactionis described in detail in copending patent application Ser. No. 436,432,filed Jan. 25, 1974, and assigned to the present assignee, and is thusnot a necessary part of this description. However, to aid thepractitioner, a brief summary of that technique is as follows.

The reactants, SiCl₄ and NH₃ of the desired purity (at least 99.9percent) are introduced into the reaction apparatus at a point where thetemperature is at least 1100° C. The temperature of the reaction zoneshould be maintained between 1100° C. and 1350° C.

A suitable diluent gas, provided it is of the requisite purity and isnon-reactive, may be present in the reaction chamber. The gas pressurein the reaction zone should be at least atmospheric in order for theproduct to be produced in particulate form. Typical diluent gasesinclude nitrogen, argon and helium. Since the finely divided product issusceptible to oxidation, the diluent gas or other inert atmosphere orvacuum should be maintained in contact with the product until it iscooled to below at least 100° C.

The mole ratio of NH₃ to SiCl₄ should preferably be maintained withinthe range of 1.25 to 15.

The NH₄ Cl by-product may be removed from the product in a separate stepby heating in a non-oxidizing atmosphere or vacuum above about 340° C.,the sublimation temperature of NH₄ Cl.

Some oxygen, typically less than about 4 weight percent of the productmay also be adsorbed on the powder, and may be reduced to less thanabout 1 percent by heating the powder in dry nitrogen or othernon-reactive atmosphere.

The silicon nitride (Si₃ N₄) powder product obtained by the above methodis characterized by an amorphous structure. Crystallization of thisstructure may be achieved by heating the powder in a refractorynon-reactive container at a temperature of from 1500° C. to 1600° C. forseveral hours, for example, from 3 to 8 hours. In accordance with theinvention, it has been discovered that carrying out this crystallizationheat treatment while the powder is in intimate contact with atitanium-containing material enhances the rate of crystallization of theSi₃ N₄ powder. The amount of titanium containing material is notcritical, as little as 0.01 weight percent having been observed topermit substantial crystallization of the powder at temperatures as lowas 1400° C. Larger amounts can be used, and the upper limit will bedictated by the point at which desired enhancement of crystallizationrate is offset by the decreased purity of the Si₃ N₄ product. Based uponthese considerations, it is preferred to maintain the titanium in theamount of from about one half to five weight percent of the powder.

A preferred method of achieving the intimate contact of the titaniumcontaining material with the Si₃ N₄ powder is to codeposit TiN with theSi₃ N₄ in accordance with the technique described above. While TiCl₄ iscompletely miscible with SiCl₄, the volume ratio of SiCl₄ to TiCl₄ mustbe about 10 to 12 times more than that which would correspond to thedesired weight ratio of Si₃ N₄ to TiN in the final product due to thefact that vapor pressures and molecular weights of SiCl₄ and TiCl₄ aredifferent.

In the following example a homogenous mixture of Si₃ N₄ and TiN wasprepared by codeposition from the vapor phase as described above andsubsequently given a crystallization heat treatment.

EXAMPLE I

An amorphous Si₃ N₄ powder containing about 3 weight percent TiN andcodeposited from the vapor phase reaction with NH₃, is heated at about1400° C. in dry N₂ for about 2 hours. The product is identified by x-raydiffraction as about 60 weight percent crystalline, about 97 weightpercent of which is the α polymorph and about 3% is the β polymorph, anda trace amount of TiN.

A pure amorphous Si₃ N₄ powder containing no TiN and deposited as above,is heated at about 1400° C. in dry N₂ for about 2 hours. X-ray analysisreveals no crystalline phases present.

As may be seen from the above example, the presence of TiN resulted incrystallization of the Si₃ N₄ powder at a significantly lowertemperature than the 1500° C. to 1600° C. required without the presenceof TiN.

In accordance with a second embodiment of the method of the invention,the intimate contact between the titanium containing material and Si₃ N₄powder may be achieved during the crystallization heat treatment bycarrying out such heat treatment in a refractory inert vessel of amaterial containing sufficient titanium in a diffusable state to resultin the leaching and diffusion of titanium from the walls of thecontainer into the Si₃ N₄ powder. The following two examples illustratethe effectiveness of such a technique by comparing crystallization heattreatments carried out in a tungsten crucible containing no titanium andin a molybdenum crucible containing titanium as an alloy component inthe amount of 0.01 to 0.05 weight percent.

EXAMPLE II

The pure amorphous Si₃ N₄ powder of Example I is heated in a tungstencrucible containing no TiN under dry N₂ at about 1500° C. for about 4hours. X-ray analysis reveals that about 5 to 10 percent crystallinematerial is present.

EXAMPLE III

The pure amorphous Si₃ N₄ powder of Example I is heated in a molybdenumcrucible containing about 0.03±0.02 weight percent titanium under dry N₂at about 1450° C. for about 2 hours. X-ray analysis reveals that about60 percent crystalline material is present.

As may be seen from the above examples carrying out the crystallizationheat treatment in the presence of a titanium containing materialsignificantly lowers the temperature at which crystallization may becarried out.

While there has been shown and described what are at present consideredthe preferred embodiments of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the scope of the invention as defined bythe appended. claims.

What is claimed is:
 1. A method for enhancing the crystallization rateof high purity fine grain amorphous Si₃ N₄ powder, the method comprisingcarrying out a crystallization heat treatment of the powder while thepowder is in intimate contact with a titanium containing material. 2.The method of claim 1 wherein the intimate contact of the Si₃ N₄ powderwith the titanium containing material is achieved by the coprecipitationof Si₃ N₄ and TiN formed by the vapor phase reaction of SiCl₄ and TiCl₄with NH₃.
 3. The method of claim 1 wherein the intimate contact of theSiCl₄ powder with the titanium containing material is achieved bycarrying out the crystallization heat treatment while the powder is incontact with the surface of a refractory inert material containingtitanium in a diffusable state.
 4. The method of claim 1 in which thetitanium containing material in intimate contact with the Si₃ N₄ powderis present in an amount corresponding to at least 0.01 weight percenttitanium of the total weight of the powder.
 5. The method of claim 4 inwhich titanium is present in the amount of from about one half to fiveweight percent.